Adjustable electrical resistor having a helical coil of resistance material in threaded, biased engagement with a rotatable internal contact member

ABSTRACT

A helical resistance element of wire sufficiently stiff to make the element self-supporting is traversed by a contact member threadedly engaging the inside of the helix at circumferentially spaced points. Contact is maintained by the fact that the member is a tight fit in the element, actually expanding it somewhat from its normal diameter. The contact member is mounted on a conductive shaft, for which a low friction, high pressure and high conductivity bearing of silver graphite material is provided at one end of the helix, external connection to the contact member being made through the bearing and the shaft. Assembly is done by temporarily expanding the end turns of the coil to permit insertion of the associated parts, which are retained by the spring contraction of the coil when released.

United States Patent 11 1 Johnston 14 1 Nov. 6, 1973 1 ADJUSTABLEELECTRICAL RESISTOR 2,899,661 8/1959 Gruer 338/202 HAVNG A HELICAL COIL0F RESISTANCE 2,680,896 6/1954 Groce 308/D1G. 10 8221;: :122? 2188ENGAGEMENT WITH A ROTATABLE 3:427:548 2/1969 DetWell61'.... 338/143INTERNAL CONTACT MEMBER 1,438,753 12/1922 Douglass 338/202 Inventor:Samuel A. Johnston, Fontana, Wis.

Bunker Ramo Corporation, Oak Brook, 111.

Filed: Dec. 27, 1971 Appl. No.: 212,686

Related US. Application Data Continuation-impart of Ser. No. 108,503,Jan. 21, 1971, abandoned.

Assignee:

US. Cl 338/143, 338/180, 338/202, 338/304 Int. Cl [1016 5/02 Field ofSearch 308/DIG. 10, 73; 200/166 C; 310/90; 338/136, 143, 147-149, 160,162, 171,202, 68, 75, 98, 118,148,157, 176, 180, 296, 304, 315, 330

References Cited UNITED STATES PATENTS Mairs 338/143 PrimaryExaminer-Robert K. Schaefer Assistant Examiner-Gerald P. TolinAttorneyFrederick M. Arbuckle [57 ABSTRACT A helical resistance elementof wire sufficiently stiff to make the element self-supporting istraversed by a contact member threadedly engaging the inside of thehelix at circumferentially spaced points. Contact is maintained by thefact that the member is a tight fit in the element, actually expandingit somewhat from itsnormal diameter. The contact member is mounted on aconductive shaft, for which a low friction, high pressure and highconductivity bearing of silver graphite material is provided at one endof the helix, external connection to the contact member being madethrough the bearing and the shaft. Assembly is done by temporarilyexpanding the end turns of the coil to permit insertion of theassociated parts, which are retained by the spring contraction of thecoil when released.

12 Claims, 7 Drawing Figures PATENTED NW 6 I975 mm NN ln'ventor SamuelAJ'ohnston ADJUSTABLE ELECTRICAL RESISTOR HAVING A HELICAL COIL OFRESISTANCE MATERIAL IN THREADED, BIASED ENGAGEMENT WITH A ROTATABLEINTERNAL CONTACT MEMBER CROSS-REFERENCE TO RELATED APPLICATION Thisapplication is a continuation-in-part of US. Pat. application Ser. No.108,503, filed Jan. 21, 1971, now abandoned.

BACKGROUND OF THE INVENTION Many applications in the electrical andelectronic fields require a variable resistance to permit adjustment ofa circuit, usually to bring its operating characteristics within certaindesired parameters. Such requirements may arise in many types ofcontrol, logic, and computer circuitry, for example. The proliferationof such circuits in present day technology, and the accompanying needfor large quantities of variable resistances, have brought forth manydevices for the purpose, but there is always a demand for lower cost,less bulk, greater stability, simpler mounting, and, for a unit of givenspace needs, greater power dissipation capability.

SUMMARY OF THE INVENTION The present invention responds to all thosedemands.

In a preferred embodiment, the present invention provides the resistanceelement as a helically wound coil of solid wire. A conductive contactmember threadedly engages the inside of the helical coil, its electricalcontact therewith being maintained by the fact that it is a tight fitwithin the coil, the contact pressure therefore being maintained by thespring character of the coil itself. Rotation of the contact member, andhence its movement along the length of the resistance element, is madepossible by mounting it on a conductive shaft supported by a conductivebearing disposed at one end of the coil, external electrical connectionto the. contact member being made through the bearing and the shaft. Thebearing is of predetermined design and of specially chosenself-lubricating material of high electrical conductivity so as topermit obtaining high pressure, low friction and low resistance contactwith the shaft while also providing for ease and reliability in shaftadjustment. At the ends of the helix, extensions of the wire from whichit is wound serve both as terminals and as supports, the wire being ofsuch size and stiffness that the helix maintains its shape whensupported by the extensions.

There is thus achieved in the preferred embodiment disclosed a variableresistance involving a minimum number of parts, and quite simple toassemble, so that the prime object of low cost is attained. Further,there is no housing, enclosure, or accessory mounting hardware required,and these facts point to the attainment of several more of the desiredobjects less space is needed, and the mounting is simple. Since ambientair can circulate'freely around the open helix, power dissipationcharacteristics are good, and by the avoidance of hotspots, theresistance value tends to be stable.

' BRIEF DESCRIPTION OF THE DRAWING Details of the arrangement by whichthese objects are attained are more fully set forth in the followingdescription and-illustrated in the accompanying drawing, in which: 1

FIG. 1 is a side elevational view, partly in section, of a preferredembodiment of the invention;

FIG. 2 is an elevational view of one end of the device, on the samescale as FIG. 1;

FIG. 3 is a side elevational view of a pair of blocks which form asupport bearing;

FIG. 4 is an end elevational view of the blocks of FIG.

FIG. 5 is an end elevational view of a contact member;

FIG. 6 is a side elevational view of the contact member of FIG. 5;

FIG. 7 is an elevational view, similar to FIG. 2, but on a somewhatenlarged scale, to illustrate the action of the contact in spreading acoil of the resistance element.

DESCRIPTION OF THE PREFERRED EMBODIMENT lnthe drawing, the referencenumeral 10 is used to indicate generally a resistance element. It is acoil wound from resistance wire, primarily in the shape of a singlelayer open helix ll, i.e., one with some space left between adjacentturns of the wire. At the ends of the helix, the wire is brought out toform the terminals 12 and 14, by which the resistance element can bemounted. In the substrate 16 of a printed circuit, for example, thewires can be thrust through holes 18 in the substrate and soldered inplace by well-known means. The terminal wires can be swedged as at 20,to form an enlargement which will abut against the substrate, to holdthe resistance element at a desired elevation above the substrate.

A contact member 22 is formed with peripheral grooves 24 in a helicalpattem,-such as to engage the inside surface of the wire helix 1 l. Thecontact member is formed of suitable electrically conductive material,such as carbon, graphite or powdered metal, and can be molded bywell-known techniques. Because of unavoidable imperfections in thehelix, such as eccentricity, out-of-roundness, or variations in pitch,the groove 24 is preferably not continuous, but the contact member isrelieved at several places around its circumference, as at 26, so thatit does not contact the wire at these places. This leaves groove 24 indiscrete sections of limited circumferential extent. Three suchsections, equally spaced circumferentially, are preferred.

The contact member is mounted on an operating shaft 30, which has aportion of reduced diameter 31 on which the contact member is retainedby swedging a rivet-like head as at 33. The shaft is formed of aconductive material, and the attachment between the shaft and thecontact member must provide for good electrical continuity. Soldering orwelding can be used where the members are metallic, and conductiveadhesives can be used when the contact member is of carbon or graphite.The shaft is provided, at its end opposite the contact member, with aslot 32 permitting its rotation by a screwdriver or similar tool.

At the end of the helix through which the operating shaft passes, a pairof bearing blocks 34 and 36 are supported in the last turn of the coil.The bearing-blocks are formed, in effect, as the two parts of a spoolsplit along a diameter. To facilitate assembly within the coil, it isadvantageous to form the parts as somewhat less than half circles, asindicated by the space across the diameter in FIG. 4, where the outlineof the parts is a constituted when the blocks 34 and 36 are located inthe relation shown in FIG. 4, one end flange of the spool, designated at38 and 40, is substantially continuous, while the other end flange has ahelical groove 42, 44, formed therein, of pitch and diameter to fit theinside of the helix 1 l, and leading into the circular groove 45, 47.The final turn of the resistance element is formed with about a halfturn which is a plane circular form, i.e., that portion of the coilbetween the crosssection at 46 in FIG. 1 and the terminal 12 iscircular, not helical. For assembly of the parts 34 and 36 within thecoil, it may be necessary to spring the last turn of the coiltemporarily to a larger diameter, by holding the main portion 11 of thehelix in a suitable fixture, suggested as shaped clamp blocks 48 shownin dotted lines in FIG. 2, and moving the terminal 12 in the directionof the arrow 51 in that figure.

The blocks 34 and 36 are provided with notches 52 in order to assurepositive high pressure contact in limited areas only between the blocksand the shaft 30, which is shown dotted in FIG. 4.

The end of the resistance element opposite the bearing blocks iscontinued in the helical form to the point where the terminal 14diverges from the helix. This permits the contact member to be assembledinto the resistance element by threading it into the coil at that end.The contact member must be a close fit in the helix,

and the bearing blocks must be a pressure fit on the v shaft. Therefore,for assembly it may be desirable to use the technique previouslydescribed, i.e., to hold the main portion 11 of the helix in clampblocks 48. The terminal 12 can then be moved in the direction 51 (FIG.2) to open up the left end turn of the coil (as seen in FIG. 1) thusincreasing the available diameter for the blocks 34 and 36, andpermitting the slotted end of the shaft to be inserted between them.Similarly, the right end turn of the coil (as seen in FIG. 1) can beopened up by moving the terminal 14 in the direction 50 (FIG. 2) topermit the contact member to be threaded into the coil.

After the terminal 12 is released, and the coil seeks to return to itsnormal diameter, the constriction upon the blocks 34 and 36 will causethem to have a firm pressure engagement with the shaft 30.

After the terminal 14 is released, there will be a similar pressureengagement between the helix 11 and the contact member 22. Wherever thecontact member may be traversed along the helix, this pressureengagement will be maintained, because the contact actually expands thehelix somewhat at the point where it is located. This expansion isillustrated (exaggerated) by the dotted lines 56 in FIG. 7.

After the contact member has been assembled within the helix, the lattermay be swedged as at 54, adjacent to the terminal 14, to provide a stopwhich will prevent the contact member from being inadvertently run outof the helix in subsequent adjustments.

It will be observed that the preferred embodiment of this inventionprovides a rheostat with only five parts,

of relatively simple construction, fulfilling the needs,

previously described. Contact pressure, both on the contact member andon the bearing block, is maintaincd without the need for any springelement other than the resistance coil itself.

Some changes may be made in the form and arrangements of the invention,yet still be within the intent and 4. purport of the following claims.For example, although a housing, enclosure or other accessory mountingor terminal structure are not required in the preferred embodiment ofthe invention disclosed herein, such additional structure may beprovided where desirable or appropriate in a particular application.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A rheostat comprising a self-supporting helical coil of resistancewire, portions of said wire diverging from said coil at the ends thereofand forming supporting terminals, a contact member threadedly engagingthe inside of said helical coil in a manner so that the position of saidcontact member in said coil may be varied by threaded movement of saidcontact member relative to said coil along the length thereof, saidcontact member engaging the coil at a plurality of discretely separatedareas within one turn thereof, an operating shaft for said contactmember extending axially of said coil and being operative to rotate saidcontact member to cause the position of said contact member in said coilto be varied as a result of threaded movement of said contact memberrelative to said coil, and a conductive bearing for said shaft retainedin one end of said coil, said conductive bearing including a pair ofconductive blocks of self-lubricating silver graphite material havingaxially extending notches for providing pressure engagement between saidblocks and said shaft at a plurality of spaced high pressure contactareas around said shaft wherein said conductive blocks are held inpressure contact with said shaft by the diametral contraction of saidcoil as a spring upon said blocks.

2. A rheostat in accordance with claim 1, wherein said portions of saidwire are swedged to provide stop means whereby said helical coil will belocated at a predetermined minimum distance above a mounting surface. i3. A rheostat in accordance with claim 1, wherein said helical coilcomprises a self-supporting single layer coil, wound in spaced helicalform with the exception of a segment of less than one turn at one end ofsaid coil, which segment is in plane circular form, said bearing beingretained in the end of said coil having said segment of plane circularform in a manner so as to provide said diametral contraction.

4. A rheostat in accordance with claim 3, wherein said blocks are placedtogether along a diameter to form a spool having a plane, circularcircumferential groove adapted to fit into said coil with said segmentthereof engaging said groove.

5. A rheostat in accordance with claim 4, wherein the wire of said coilat the opposite end from said segment is deformed to form a stopengageable with said contact member, preventing overtravel of saidcontact memher.

6. A rheostat in accordance with claim 3, wherein said blocks are placedtogether along a diameter to form a spool having plane circularcircumferential groove, and having a helical groove leading into saidplane circular groove, said plane circular groove being adapted to fitinto said coil with said segment thereof engaging said plane circulargroove, and said helical groove being adapted to engage a helicalportion of said coil adjacent to said segment.

7. A variable resistance comprising a helical coil of wire of sufficientsize and stiffness so as to be selfsupportable, a conductive contactmember biased against the inside of said coil and including positionvarying means threadedly engaging, the inside of said coil so as toprovide for varying the axial position of said contact member in saidcoil by threaded relative movement therebetwcen, an operating shaftcoupled to said position varying means for varying the axial position ofsaid contact member, said shaft extending axially of said coil and beingoperative to rotate said contact member to cause the position of saidcontact member in said coil to be varied as'a result of threadedmovement of said contact member relative to said coil, a conductivebearing for said shaft secured at one end of said coil, said conductivebearing comprising a pair of oppositely disposed bearing blocks ofhighly conductive self-lubricating material biased against said shaftand held in pressure contact therewith, said conductive bearingincluding a pair of conductive blocks of selflubricating silver graphitematerial having axially extending notches for providing pressureengagement between said blocks and said shaft at a plurality of spacedhigh pressure contact areas around said shaft, and means for providingexternal connection to said variable resistance such that the resistanceprovided thereby is dependent upon the axial position of said whereinsaid pair of bearing blocks are formed as the two parts of a spool splitalong a diameter and having opposed notches shaped to provide highpressure contact of said blocks with said shaft.

11. The invention in accordance with claim 7, wherein said blocks haveopposing notches shaped to provide high pressure engagement of saidblocks with said shaft in a limited plurality of areas.

12. The invention in accordance with claim 11, wherein said notches areof triangular shape so that said blocks engage said shaft at fourdiscretely separated areas.

1. A rheostat comprising a self-supporting helical coil of resistancewire, portIons of said wire diverging from said coil at the ends thereofand forming supporting terminals, a contact member threadedly engagingthe inside of said helical coil in a manner so that the position of saidcontact member in said coil may be varied by threaded movement of saidcontact member relative to said coil along the length thereof, saidcontact member engaging the coil at a plurality of discretely separatedareas within one turn thereof, an operating shaft for said contactmember extending axially of said coil and being operative to rotate saidcontact member to cause the position of said contact member in said coilto be varied as a result of threaded movement of said contact memberrelative to said coil, and a conductive bearing for said shaft retainedin one end of said coil, said conductive bearing including a pair ofconductive blocks of self-lubricating silver graphite material havingaxially extending notches for providing pressure engagement between saidblocks and said shaft at a plurality of spaced high pressure contactareas around said shaft wherein said conductive blocks are held inpressure contact with said shaft by the diametral contraction of saidcoil as a spring upon said blocks.
 2. A rheostat in accordance withclaim 1, wherein said portions of said wire are swedged to provide stopmeans whereby said helical coil will be located at a predeterminedminimum distance above a mounting surface.
 3. A rheostat in accordancewith claim 1, wherein said helical coil comprises a self-supportingsingle layer coil, wound in spaced helical form with the exception of asegment of less than one turn at one end of said coil, which segment isin plane circular form, said bearing being retained in the end of saidcoil having said segment of plane circular form in a manner so as toprovide said diametral contraction.
 4. A rheostat in accordance withclaim 3, wherein said blocks are placed together along a diameter toform a spool having a plane, circular circumferential groove adapted tofit into said coil with said segment thereof engaging said groove.
 5. Arheostat in accordance with claim 4, wherein the wire of said coil atthe opposite end from said segment is deformed to form a stop engageablewith said contact member, preventing overtravel of said contact member.6. A rheostat in accordance with claim 3, wherein said blocks are placedtogether along a diameter to form a spool having plane circularcircumferential groove, and having a helical groove leading into saidplane circular groove, said plane circular groove being adapted to fitinto said coil with said segment thereof engaging said plane circulargroove, and said helical groove being adapted to engage a helicalportion of said coil adjacent to said segment.
 7. A variable resistancecomprising a helical coil of wire of sufficient size and stiffness so asto be self-supportable, a conductive contact member biased against theinside of said coil and including position varying means threadedlyengaging the inside of said coil so as to provide for varying the axialposition of said contact member in said coil by threaded relativemovement therebetween, an operating shaft coupled to said positionvarying means for varying the axial position of said contact member,said shaft extending axially of said coil and being operative to rotatesaid contact member to cause the position of said contact member in saidcoil to be varied as a result of threaded movement of said contactmember relative to said coil, a conductive bearing for said shaftsecured at one end of said coil, said conductive bearing comprising apair of oppositely disposed bearing blocks of highly conductiveself-lubricating material biased against said shaft and held in pressurecontact therewith, said conductive bearing including a pair ofconductive blocks of self-lubricating silver graphite material havingaxially extending notches for providing pressure engagement between saidblocks and said shaft at a plurality of spaced high Pressure contactareas around said shaft, and means for providing external connection tosaid variable resistance such that the resistance provided thereby isdependent upon the axial position of said contact member in said coil,and such that external connection to said contact member is made throughsaid bearing and said shaft, wherein said conductive blocks are held inpressure contact with said shaft by the diametral contraction of saidcoil as a spring upon said blocks.
 8. The invention in accordance withclaim 7, wherein silver constitutes the major proportion of said silvergraphite material.
 9. The invention in accordance with claim 8, whereinsaid silver graphite material is 93 percent silver.
 10. The invention inaccordance with claim 7, wherein said pair of bearing blocks are formedas the two parts of a spool split along a diameter and having opposednotches shaped to provide high pressure contact of said blocks with saidshaft.
 11. The invention in accordance with claim 7, wherein said blockshave opposing notches shaped to provide high pressure engagement of saidblocks with said shaft in a limited plurality of areas.
 12. Theinvention in accordance with claim 11, wherein said notches are oftriangular shape so that said blocks engage said shaft at fourdiscretely separated areas.